1. Field of the Invention
The present invention generally relates to crystalline colloidal arrays (CCAs) comprising core-shell particles composed of an inorganic, high refractive index core and a highly charged polymeric layer surrounding the core. The present invention further relates to methods of making the same. The invention also relates to optical filters, optical coatings, cosmetics and hydrogel-based sensors and devices comprising these CCAs that utilize their diffraction properties.
2. Background Art
Crystalline Colloidal Arrays (CCAs) are three-dimensional arrays of highly monodisperse particles that self assemble into body-centered cubic (BCC) or face-centered cubic (FCC) lattices and Bragg-diffract light according to the lattice spacing and the refractive index ratio between the spheres and the matrix.
Highly monodisperse polymer spheres prepared by emulsion polymerization have been mainly utilized as the high refractive index components of CCAs due to their high monodispersity and high surface charge. (See Holtz, J. H. and Asher, S. A., Nature 389:829-832 (1997) and Asher, S. A., Chapter in Nanoparticles: Building Blocks for Nanotechnology, Rotello, V. M., Ed., Kluwer, N.Y., 2004.) Such lattices have a low refractive index mismatch with the surrounding water, leading to modest diffraction efficiency. In order to increase the refractive index mismatch, the inverse opal technique has been proposed which utilizes a high refractive index matrix (such as TiO2) encompassing air spheres, giving rise to a high refractive index ratio between the spheres and the matrix. Although this method yields highly diffracting media, it encompasses several disadvantages: 1) the lattices lack mechanical stability due to annealing deformations at high temperature; 2) the lattices have high defect density due to annealing and etching defects; 3) the annealed material is highly rigid, thus biologically compatible molecules cannot be readily introduced into the array for sensor applications; and 4) the high refractive index of the material is usually introduced by costly methods such as sputtering.
Zinc sulfide (ZnS) is a high refractive index material which has a bulk refractive index of between 2-3. (ZnS being 2.3-2.55 at the near IR-Visible regions). Moreover, ZnS has no absorption throughout the visible and infrared regions (λ between 0.37 μm and 14 μm) making it a good candidate for CCA applications.
U.S. Pat. Nos. 6,433,931 and 6,671,097 describe the use of zinc sulfide as a dielectric contrast-enhancing additive to alternate the refractive index of a polymeric material for photonic crystal applications.
Since the original work of Wilhelmely and Matiević published in Chem. Soc. Faraday Trans. 1, 80: 563 (1984) on precipitating ZnS particles from a homogenous solution of thioacetamide (TAA) and zinc nitrate, a number of investigators have utilized this procedure to prepare monodisperse microparticles in water. (See Celikkaya et al., Americ. Cer. Soc. 73: 245 (1990); Celikkaya et al., Amer. Cer. Soc. 73: 2360 (1990); Vacassy et al., Amer. Cer. Soc. 81:2699 (1998); Nomma et al., J. Colloid Interface Sci. 223:179 (2000); and Duran et al., Colloid Polym Sci. 93:215 (1993)). Further investigations of the reaction mechanism, electrophoretic properties and kinetic studies have also been preformed. (See, Bayer et al., Mater. Chem. 12:2940 (2002); Mencan et al., Faraday Disscuss. 122:203 (2002); Duran et al., J. Colloid Interface Sci. 173:436 (1995); and Eshuis et al., J. Colloid Polym. Sci. 272:1240 (1994)). Eshuis et. al. proposed a mechanism that involves a Brownian coagulation of highly monodisperse, primary nanoparticles, followed by the formation of perfect spheres when the initial particles reach a certain size, e.g., up to ˜50 nm. Several parameters were found to play significant roles in controlling the monodispersity and size of the precipitated particles. These are, the reaction temperature, the ratio of thioacetamide (TAA):Zn, and the absolute Zn2+ source concentration. Furthermore, agitation, pH and purity were also found to be important for preparing monodisperse particles.
Besides precipitating ZnS from dilute aqueous solutions using a variety of methods along the path of Matiević's original work, efforts were also taken to grow ZnS particles in concentrate media. Sugimoto et al. have proposed to use the “sol-gel” method to produce monodisperse ZnS particles from concentrated solutions of zinc-chelate complexes. (See Sugimoto et al., J. Colloid Interface Sci. 180:305 (1996) and Sugimoto et al, Colloids and Surfaces, A. 135: 207 (1998)). Zinc sulfide nuclei form immediately after the mixing of the sulfur source (TAA) with the zinc source solutions. Within a few minutes, the nucleation is completed. Zinc cations slowly released thereafter from the zinc-chelate complexes would react with sulfide anions decomposed from TAA to form ZnS and grow onto the nuclei to form monodisperse ZnS particles. By controlling the composition and concentration of the Zn-chelate solution and the reaction time, it is possible to produce a high yield of monodisperse zinc sulfide particles of controlled sizes.
Van Blaaderen and co-workers proposed the use of core shell ZnS:SiO2 particles as building blocks for photonic crystals. (See, Velikov et al., Appl. Phys. Lett. 80:40 (2002) and Velikov et al, Langmuir 14:4779 (2001)). According to the synthesis procedure described by these authors, most of the zinc sulfide particles were formed during secondary aggregation. Thus the yield is low and the possibility to control the particle size is limited. Liddell and co-workers proposed using non-spherical ZnS colloidal building blocks (>500 nm) for three-dimensional photonic crystals and investigated the formation of dimers, trimers and tetramers in these systems. (See, Liddell et al., J. Colloid Interface Sci. 274:103 (2004); and Liddell et al., Materials Characterization 50:69 (2003)).
Thus, there is a need for preparing high refractive index (n>1.8) monodisperse spheres with sufficient surface charge to allow their self assembly into crystalline colloidal arrays. Therefore, core-shell particles need be made, so that the shells might be sufficiently charged to enable the particles to form CCA.
It is possible to coat core particles of micro to nano sizes with several methods. One path is interfacial polymerization, as has been performed on SiO2 particles. (See Bourgeat-Lami et al., J. Colloid Interface Sci., 197: 293 (1998) and Xu et al., J. Am. Chem. Soc., 126: 7490 (2004)). However, it is impossible to apply this method directly to ZnS particles, as the oligomers in the solution would not attach to the ZnS surface. Another path is to coat colloidal particles with alternating layers of positively and negatively charged polyelectrolytes which are polymers with groups capable of ionic dissociation. (See Sukhorukov et al., Colloids Surfaces A, 137: 253 (1998) and Sukhorukov et al., Polymers for Advanced Technologies, 9: 759 (1998)). U.S. Pat. No. 7,101,575 has used this method as an intermediate step to make hollow capsules. However, although particles may be coated this way in aqueous solutions, they do not readily form crystalline structures, as the repulsive forces between the particles are usually insufficient for the particles to stabilize, either due to insufficient surface charge, or due to the screening of salt ions and other impurities. Even if such a crystalline order could temporarily form, it usually decays very quickly. This occurs because the polyelectrolyte chains are only bound to the particle surfaces via electrostatic forces with the aid of additional salt ions. As these salt ions gradually diffuse out of the coated shells, the ionic strength of the particle suspension increases with time. As the CCA is only stabilized by electrostatic forces between the charged surfaces, the increase of ionic strength screens the repulsive forces between particles and causes the order to decay. When too many ions inside the coated shells permeate out and become free ions, the coated layers themselves also fall off. This is especially true with single layer coatings. It had been suggested that continuous washing and time wear also causes polyelectrolyte chains to fall off multi-layer shells. Additionally, multi-layer coating induces bridging of different shells, which is fatal for the formation of CCA.
In summary, there is a need for preparing highly charged, highly monodisperse core-shell particles with high refractive index (n>1.8) that can self assembly into crystalline colloidal arrays.
There is a need for novel, high refractive index, highly monodisperse, highly charged ZnS or ZnO particles that will self assemble into crystalline colloidal arrays and Bragg-diffract light in the visible and near-IR regions. There is also a need for providing a route for the synthesis of such monodisperse ZnS or ZnO particles. There is also a need for providing a method for effectively coating the particles with highly charged shells with long durability and stability, without either bridging between shells or stripping-off of the shells due to time wear. Crystalline colloidal arrays formed by such highly charged, highly monodisperse, high refractive index particles can be useful in photonic crystal applications where a large refractive index contrast is desired in order to increase the diffraction efficiency for coating, filtering and sensing applications. Thin photonic crystal films made of these spheres can result in essentially 100% light diffraction for films <20 μm thick. Thus, such CCAs have use inter alia in optical filters, optical coatings, hydrogel-based sensors and devices that use the diffraction properties of CCAs.